Boost control may be used at various engine operating conditions to improve engine performance. For example, various approaches may be used to control turbine speed and exhaust manifold pressure at varying operating conditions to manage boost.
One example approach is shown by Styles et al. in US 2011/0302917 wherein the boosted engine system includes a twin-scroll turbocharger. Therein, flow through the different scrolls of the turbocharger is separated or combined, via valve adjustments, based on operating conditions to provide boost and engine speed control.
However, the inventors herein have identified potential issues with such an approach. Specifically, turbocharger actuator adjustments used to provide engine control may have a torque impact on the engine. For example, an actuator adjustment that combines flow through the different scrolls results in a decrease in exhaust manifold pressure upstream of the turbine inlet(s). This decrease in exhaust manifold pressure results in additional fresh air being drawn into, and trapped within, engine cylinders on subsequent engine cycles. When the increase in airflow is matched by fuel to maintain a constant air-fuel ratio and constant ignition timing, a transient torque disturbance results which leads to poor engine driveability. Likewise, an actuator adjustment that separates flow through the different scrolls results in an increase in exhaust manifold pressure upstream of the turbine inlet(s). This increase in exhaust manifold pressure reduces the drawing in of fresh air into the engine cylinders on subsequent engine cycles. When the decrease in airflow is matched by reduced fuel to maintain a constant air-fuel ratio and constant ignition timing, a torque dip results which also leads to poor engine driveability. In both cases, the torque disturbance may degrade the drive experience of the vehicle operator.
The inventors herein have recognized the above issues and provided methods of overcoming the torque disturbances by operating a boosted engine system having a binary flow turbine. In one example, the torque impact can be reduced by a method for an engine comprising: transitioning a restriction in exhaust upstream of a first scroll of a multi-scroll exhaust turbine based on operating conditions, while adjusting an engine actuator during the transition to maintain engine torque. In addition, a timing of the transitioning may be adjusted based on a transmission event. In this way, the torque impact may be better masked.
In one example, an adjustment to a scroll valve coupled to only an outer scroll of a multi-scroll exhaust turbine may be determined based on engine operating conditions. For example, during conditions where turbine spool-up is required, the scroll valve may be scheduled to be closed for a duration. A torque impact associated with the scroll valve may be determined. Then, adjustments to one or more engine actuators that may compensate for the torque impact may be concurrently scheduled. For example, during conditions where the scroll valve is closed to increase exhaust manifold pressure, the drop in airflow to the engine cylinders (and resulting torque dip) may be counteracted by a transient increase in intake throttle opening. As another example, during conditions where the scroll valve is opened to decrease exhaust manifold pressure, the rise in airflow to the engine cylinders (and resulting torque disturbance) may be counteracted by a transient decrease in intake throttle opening. Other engine actuators that may be adjusted include, for example, VCT, valve timing, valve overlap, fuel injection, spark timing, wastegate opening and EGR valve opening. As such, this helps to overcome at least a part of the torque impact of the scroll valve adjustment.
In addition, a timing of the scroll valve transition may be adjusted based on the shift schedule of a transmission coupled to the engine. For example, if an upcoming transmission event is expected (e.g., a transmission upshift or downshift), the scroll valve transition may be timed to at least partially overlap (if possible) the transmission event. For example, the scroll valve may be transitioned during a transmission upshift or immediately after the upshift. By timing the scroll valve transition based on the transmission event, the torque impact is better masked. In addition, one or more transmission clutches may be slipped during the scroll valve transition to further reduce the torque impact of the scroll valve adjustment.
In this way, scroll valve adjustments may be advantageously used to provide engine speed control and improve boost response at various engine operating conditions. By adjusting one or more engine torque operators based on the scroll valve adjustment, the torque impact of the scroll valve adjustment is reduced. By also timing the scroll valve adjustment to coincide with a transmission event, the torque impact is better masked. Overall, the torque impact of the scroll valve adjustment experienced by the vehicle operator is reduced and vehicle driveability is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.